Self-driven synchronous rectified boost converter with inrush current protection using bidirectional normally on device

ABSTRACT

A boost converter in which the conventional boost diode is replaced by a bidirectional normally conducting semiconductor switch. The circuit may be implemented so the bidirectional switch is self-driven by connecting a low voltage Schottky diode between a first gate-source terminal pair. An inrush current protection function may be provided by utilizing a second gate-source terminal pair to turn the switch on and off independent of the self-driven operation in response to predetermined excessive load current conditions. The inrush current protection function is implemented by use of a second Schottky diode connected between the second gate and source terminals, and an RC circuit connecting the second gate terminal to the return rail for the converter power output transistor with an externally controlled switch connected to the RC circuit to control the bias according to the load current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is reissue application of U.S. application Ser. No.11/202,134, filed Aug. 11, 2005, now U.S. Pat. No. 7,276,883, which isbased on and claims priority to U.S. Provisional Application 60/600,914,filed Aug. 12, 2004, the entire disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to synchronous rectifier boost converters,and more particularly to such devices in which the conventional boostdiode is replaced by a bidirectional semiconductor switches are used

2. Relevant Art

Power factor correction (PFC) is required by international standards(EN61000-3-2) to reduce harmonic emissions in AC powered systems. Themost common conventional solution employs an input rectifier bridge,followed by a boost switching converter, controlled by a voltage and acurrent loop. FIG. 1 shows a typical PFC rectifier stage, generallydesignated 10, including an input circuit 12, a diode bridge rectifier14 feeding a MOSFET 16 through a boost choke 18, and a boost diode 20which provides the output power through a capacitor 22. The loadcircuit, shown schematically as a resistor 24, is connected acrosscapacitor 22.

Gate control for MOSFET 16, and PFC are provided by a suitable logiccircuit 26. With this circuit, the voltage and current at the converterinput 12 will be proportional at all times, generating the desiredresistive behavior at the input of the system.

There are typically two problems that arise in boost topologyconfigurations, namely high reverse recovery losses, and control ofinrush current at startup. As to the first problem, when MOSFET 16 turnson during normal operation, the reverse recovery charge of boost diode20 causes significant switching losses, seriously limiting the maximumswitching frequency.

The second problem typically occurs at system startup, when the outputcapacitor 22 is discharged: the output capacitor is charged by therectified AC line. The amplitude of the charging current is limited bythe impedance of the input loop, resulting in a significant inrushcurrent that can cause component failures.

In conventional topologies, there is no controllable switch in the pathof the charging current by which the current path can be shut down ifnecessary. Conventional solutions for this problem take the form ofnegative temperature coefficient (NTC) or standard resistors withrelays, SCR's, as illustrated on the input side of rectifier bridge inFIG. 1.

Specific conventional implementations of PFC in boost converters may befound in the following U.S. patents: U.S. Pat. No. 6,285,170 B1 toMatsumoto et al. for SWITCHING POWER SUPPLY; U.S. Pat. No. 5,420,780 toBernstein et al. for APPARATUS FOR LIMITING INRUSH CURRENT; U.S. Pat.No. 5,994,882 to Ma for SYNCHRONOUS RECTIFIER FOR BOOST CONVERTERS. Inall of these, however, the solutions address only one of the two mainissues (inrush and reverse recovery losses) and they all use additionalcomponents which can not conveniently be fabricated as part of theconverter IC. Clearly, a need exists for a better solution whichaddresses both problems, and also allows convenient integration.

SUMMARY OF THE INVENTION

The present invention seeks to satisfy the above-noted needs bysubstituting a bidirectional normally conducting semiconductor switchfor the boost diode shown in FIG. 1. This known device is capable ofconducting and blocking current in both directions, and is sometimesreferred to as a four quadrants switch, because it is a device capableof conducting current and blocking voltage in both directions, and thusis capable of working in the four quadrants of the VI plane. A schematicdiagram of such a device, generally denoted at 32, is shown in FIG. 2.

Here, the bidirectional current path is between a first source terminal34 and a second source terminal 36. Control is provided by bias voltagesprovided by a first voltage source 38 connected between a first gateterminal 40 and source terminal 34 and a second voltage source 42connected between a second gate terminal 44 and source terminal 36. Adevice of this kind is characterized by the fact that when a negativebias is applied across either one or both of the gate-source pairs, thedevice will be off. Only if the voltage at both gates is zero, cancurrent flow between the two source terminals.

According to a first aspect of the invention, both a synchronousrectification function and a current inrush limiting function areimplemented in a boost converter using a single bidirectional normallyon switch. Preferably, according to this aspect of the invention, theboost converter employs a self-driven topology. Advantageously, thebidirectional switch and associated circuitry will be part of the ICwhich implements the boost converter itself.

According to a second aspect of the invention, a self-driven boostconverter with inrush current limiting protection is implemented byreplacing the conventional boost diode with a normally on bidirectionalsemiconductor switch having a low voltage Schottky diode connectedbetween the gate and source terminals of the line-side pair.

According to a third aspect of the invention, inrush current limitingprotection is implemented in a boost converter by replacing theconventional boost diode with a normally on bidirectional semiconductorswitch using one (preferably) the load-side gate to turn off thebidirectional switch under control of a suitable logic circuit when theload current reaches dangerous levels. Both short circuit and overloadprotection can be provided in this manner.

Further according to the third aspect of the invention, there may beprovided a low voltage Schottky diode connected between one of the gateand source terminal pairs of the bidirectional switch, preferably theline-side pair, with a second Schottky diode connecting the load-sidegate terminal to a circuit protection logic circuit.

Circuits according to the various aspects of the invention requireminimal addition of extra components, making the device economical andeasily implemented as an IC with the rest of boost converter. Functionalimprovements over conventional practice are also obtained. For thesynchronous rectification function, only the small forward voltage ofthe low voltage Schottky diode (0.2-0.3 V) and the R_(DSon) loss of theswitch contribute to conduction losses, making the circuit veryefficient. This represents an advantage even compared to the mostadvanced wide bandgap rectifier diodes (SiC and GaAs) which exhibit highconduction losses. Also, there are no reverse recovery losses, but onlycapacitive discharge of the switch capacitance.

For the inrush current limiting function, an important advantage is thatthe current path can be opened at any given time, providing a solidstate fuse function. A complete and accurate inrush current control istherefore possible.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a schematic circuit diagram of a typical PFC rectifier stagehaving boost converter topology.

FIG. 2 is a schematic diagram representing the functionality of abidirectional normally on switch.

FIG. 3 is a simplified schematic representation of a synchronous boostconverter having inrush current limiting implemented according to thepresent invention.

FIG. 4 is a schematic diagram of a self-driven synchronous rectifierboost converter according to the present invention.

FIG. 5 is a circuit diagram of an implementation of the device of FIG. 4providing inrush current protection.

Throughout the drawings, like parts are designated by the same referencenumber.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to FIG. 3, the basic concept of a synchronous boostconverter, generally designated at 50, has the same general architectureas the conventional circuit of FIG. 1 with a diode rectifier bridge 14,a high side MOSFET 16, a boost choke 18 and an output capacitor 22, towhich a load circuit 24 is connected in parallel. The boost diode ofFIG. 1, however, is replaced by a bidirectional normally on switch 52 ofthe type described above, and illustrated in FIG. 2. Inrush andsynchronous rectification control is provided by a logic unit 54, whichmay be of any suitable or desired type, or as described in the exemplaryembodiments described below, generates the appropriate control signalsfor switch 52. MOSFET 16 is driven in conventional fashion by suitablepulse width modulation logic (not shown).

A preferred exemplary, but non-limiting implementation employs aSchottky diode 58 connected between the line side gate and sourceterminals 60 and 62 of bidirectional switch 64. In this implementation,the load side gate 66 is not driven and is therefore connected directlyto load side source 68.

Switch 64 needs to be able to block voltage in both directions. In aboost converter, when output capacitor 22 is discharged, the outputvoltage is lower than the input. However, when the circuit is operating,the output voltage is always larger than the input voltage.Correspondingly, switch 64 needs to be able to conduct current at leastin one direction.

This functionality is obtained by use of low voltage Schottky diode 58which generates the gate signal for turning on switch 64. When MOSFET 16turns on, the current will be diverted into its drain. As soon as thevoltage starts to build up across Schottky diode 58, switch 64 turnsoff, blocking the output voltage. When MOSFET 16 turns off, the oppositeprocess takes place.

As previously noted, in the circuit of FIG. 4, the second gate 66 is notused and does not affect the operation. However, because gate 66 can beused to independently control the operation of switch 64, it can be usedto implement inrush current protection, as illustrated in FIG. 5.

The inrush current protection implementation circuit 70 illustrated inFIG. 5 differs from circuit 56 in that a second Schottky diode 72 isprovided between the load side gate and drain terminals 66 and 68 ofswitch 64, and a current path to the low side rail 80 from MOSFET 16 isprovided through a series combination of resistor 74 and capacitor 76.In addition, a switch 78 is connected across capacitor 76. This isoperated by PFC logic circuit 78 to control the voltage at gate 66. Inaddition, an inrush diode 82 in series with an inrush resistor 84 mayalso be provided, as described below.

In operation, at system startup, when the AC line is applied to thesystem across bridge 14, output capacitor 22 is still discharged.Regardless of the position of switch 78, there will be 0V bias appliedto the gate 66 and therefore the switch 68 will be “on”.

As current starts flowing thru the inductor 18, diode 58 and switch 68(assuming inrush diode 82 and resistor 84 not present), voltage willstart to build up on the output capacitor 22. With the control switch 78closed, the same output voltage will be present on the clamp diode 72and therefore applied to the gate 66 of the switch 68. When the outputvoltage reaches the threshold voltage of switch 68, the switch will turnoff, blocking the charging current path.

When the control switch 78 is open, capacitor 76 will start chargingtoward the same voltage as on capacitor 22, and gate 66 will track thevoltage as capacitor 76 charges. The charging time is determined by theRC time constant of resistor 74 and capacitor 76 and can be madearbitrarily long, to limit inrush current to desired value.Alternatively, switch 78 can be pulse width modulated (PWM) to controlthe rate of rise of the input current. Inrush diode 82 and inrushresistor 84 can also be used to provide an extra path for charging theoutput capacitor 22.

Switch 78 can be closed at anytime during operation, to open the currentpath from input to output. The PFC control will usually monitor the buscurrent. When the set current limit is reached, switch 78 will beclosed, causing a negative bias on gate 66 and opening switch 68.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will be apparent to those skilled in the art in light ofthe description herein. It is intended, therefore, that the inventionnot be limited by the specific disclosure herein, but that it be giventhe full scope permitted according to the appended claims.

1. A boost converter comprising: a power output switch; an outputcapacitor coupled to the power output switch for connection to a loadcircuit; a bidirectional normally on semiconductor switch coupledbetween the output capacitor and the power output switch; a Schottkydiode connected between a first gate terminal and a first sourceterminal of the bidirectional switch; and a direct connection between asecond gate terminal and a second source terminal of the bidirectionalswitch, whereby the bidirectional switch is self driven, and provides inits conductive state, a current path between the power output switch andthe output capacitor through the first and second source terminals.
 2. Aboost converter according to claim 1, further including controlcircuitry for the bidirectional switch which implements synchronousrectification and inrush current limiting protection.
 3. A boostconverter according to claim 1, wherein the first source terminal andthe first gate terminal of the bidirectional switch are on a line sideof the bidirectional switch, and the second source terminal and thesecond gate terminal are on a load side of the bidirectional switch. 4.A boost converter according to claim 1, wherein the power output switchis a power MOSFET.
 5. A boost converter according to claim 1, whereinthe bidirectional switch and other parts of the boost converter arefabricated on a single IC.
 6. A self-driven boost converter with inrushcurrent limiting protection comprising: a power output transistor; anoutput capacitor coupled to the output transistor for connection to aload circuit; a bidirectional normally on semiconductor switch coupledbetween the output capacitor and a first side of a current path for thepower output switch; a first biasing circuit connected between a firstgate terminal and a first source terminal of the bidirectional switch; asecond biasing circuit connected between a second gate terminal and asecond source terminal of the bidirectional switch; and a controlcircuit for controlling the second biasing circuit.
 7. A converteraccording to claim 6, wherein the first biasing circuit is a Schottkydiode connected between the first gate terminal and the first sourceterminal, whereby the bidirectional switch is self driven.
 8. A boostconverter according to claim 7, wherein the second biasing circuit iscomprised of: a second Schottky diode connected between the second gateterminal and the second source terminal; a series RC circuit connectedbetween the second gate terminal and a second terminal of the currentpath for the output transistor; and a control switch operable to a firststate to permit a bias voltage at the second gate terminal to assume avalue which turns off the bidirectional switch.
 9. A boost converteraccording to claim 8, wherein the control switch operates in the firststate to bypass a capacitor in the series RC circuit.
 10. A boostconverter according to claim 8, further including a control circuit forthe second biasing circuit which is operable in response to apredetermined condition of the load current to place the control switchin the first state.
 11. A boost converter according to claim 8, whereina time constant of the RC circuit is set to determine the maximumpermitted inrush current.
 12. A boost converter according to claim 6,wherein the second biasing circuit is comprised of: a Schottky diodeconnected between the second gate terminal and the second sourceterminal; a series RC circuit connected between the second gate terminaland a second terminal of the current path for the output transistor; andan externally controlled switch operable to a first state which permitsa bias voltage at the second gate terminal to assume a value which turnsoff the bidirectional switch.
 13. A boost converter according to claim12, wherein the control switch operates in the first state to bypass acapacitor in the series RC circuit.
 14. A boost converter according toclaim 12, further including a control circuit for the second biasingcircuit which is operable in response to a predetermined condition ofthe load current to place the control switch in the first state.
 15. Aboost converter according to claim 12, wherein a time constant of the RCcircuit is set to determine the maximum permitted inrush current.
 16. Aboost converter according to claim 12, further including a diode inseries with a resistor connected between the line and load sides of thebidirectional switch.
 17. A boost converter according to claim 12,wherein the externally controlled switch is pulse width modulated tocontrol the rise of the input current.
 18. A boost converter accordingto claim 6, wherein the bidirectional switch and other parts of theboost converter are fabricated on a single IC.
 19. A boost convertercomprising: a power output switch; an output capacitor coupled to thepower output switch for connection to a load circuit; a normally onsemiconductor switch coupled between the output capacitor and the poweroutput switch; a Schottky diode connected between a first gate terminaland a first source terminal of the switch; and a direct connectionbetween a second gate terminal and a second source terminal of theswitch, whereby the switch is self driven, and provides in itsconductive state, a current path between the power output switch and theoutput capacitor through the first and second source terminals.
 20. Theboost converter according to claim 19, further including controlcircuitry for the switch which implements synchronous rectification andinrush current limiting protection.
 21. The boost converter according toclaim 19, wherein the first source terminal and the first gate terminalof the switch are on a line side of the switch, and the second sourceterminal and the second gate terminal are on a load side of the switch.22. The boost converter according to claim 19, wherein the power outputswitch is a power MOSFET.
 23. The boost converter according to claim 19,wherein the switch and other parts of the boost converter are fabricatedon a single IC.
 24. A self-driven boost converter with inrush currentlimiting protection comprising: a power output transistor; an outputcapacitor coupled to the output transistor for connection to a loadcircuit; a normally on semiconductor switch coupled between the outputcapacitor and a first side of a current path for the power outputswitch; a first biasing circuit connected between a first gate terminaland a first source terminal of the switch; a second biasing circuitconnected between a second gate terminal and a second source terminal ofthe switch; and a control circuit for controlling the second biasingcircuit.
 25. The converter according to claim 24, wherein the firstbiasing circuit is a Schottky diode connected between the first gateterminal and the first source terminal, whereby the switch is selfdriven.
 26. The boost converter according to claim 25, wherein thesecond biasing circuit is comprised of: a second Schottky diodeconnected between the second gate terminal and the second sourceterminal; a series RC circuit connected between the second gate terminaland a second terminal of the current path for the output transistor; anda control switch operable to a first state to permit a bias voltage atthe second gate terminal to assume a value which turns off the switch.27. The boost converter according to claim 26, wherein the controlswitch operates in the first state to bypass a capacitor in the seriesRC circuit.
 28. The boost converter according to claim 26, furtherincluding a control circuit for the second biasing circuit which isoperable in response to a predetermined condition of the load current toplace the control switch in the first state.
 29. The boost converteraccording to claim 26, wherein a time constant of the RC circuit is setto determine the maximum permitted inrush current.
 30. The boostconverter according to claim 24, wherein the second biasing circuit iscomprised of: a Schottky diode connected between the second gateterminal and the second source terminal; a series RC circuit connectedbetween the second gate terminal and a second terminal of the currentpath for the output transistor; and an externally controlled switchoperable to a first state which permits a bias voltage at the secondgate terminal to assume a value which turns off the switch.
 31. Theboost converter according to claim 30, wherein the control switchoperates in the first state to bypass a capacitor in the series RCcircuit.
 32. The boost converter according to claim 30, furtherincluding a control circuit for the second biasing circuit which isoperable in response to a predetermined condition of the load current toplace the control switch in the first state.
 33. The boost converteraccording to claim 30, wherein a time constant of the RC circuit is setto determine the maximum permitted inrush current.
 34. The boostconverter according to claim 30, further including a diode in serieswith a resistor connected between the line and load sides of the switch.35. The boost converter according to claim 30, wherein the externallycontrolled switch is pulse width modulated to control the rise of theinput current.
 36. The boost converter according to claim 24, whereinthe switch and other parts of the boost converter are fabricated on asingle IC.